Parallel connection of electric power devices, e.g. frequency converters, is required at very high output power level (e.g. >1 MW) due to limitations of commercial switch type power components like IGBT modules. The parallel connection may be implemented either inside a single unit by connecting several power components in parallel or connecting larger similar units in parallel. The parallel connection inside a single unit is problematic because each different power level requires its own mechanical construction. That is why normally entire power units are connected in parallel, which is advantageous also from manufacturing and service point of view. The parallel connection of power units requires normally also use of external balancing impedances, e.g. inductors, between the output terminals and common point of coupling.
Insulated gate bipolar transistors (IGBT) are switch-type power electronic components which are widely used in the main circuits of power electronic devices, like in frequency converters. IGBT is a gate controlled component, which means that it can be switched on/off by a voltage signal supplied to the gate terminal. IGBT is an ideal component for power electronic applications due to a fast response time to the gate control signal, thus making it possible to keep the load current under precise control.
In the following, IGBT is used as an example of the power semiconductor switch, but as is clear for a person skilled in the art, there exist other type of power semiconductor components, too, whereto the present invention can be applied.
The load performance (i.e. current loadability) of parallel connected power units is normally lower than the performance sum of single devices, due to that the load current does not necessarily share equally between the devices. That is why it is normal to use a derating factor, e.g. 0.9 which means thatPsum=0.9×ΣPN  [1]
where Psum is the sum output power of the parallel connected units and PN is the nominal output power of each single unit.
A derating factor means a decrease to the theoretical maximum power, being thus a problem e.g. in economic terms. It is advantageous to be able to use as high a derating factor as possible, which is possible by balancing the loads as precisely as possible.
There are many known methods for the balancing, e.g. the patent publication U.S. Pat. No. 8,188,694 B2 presents a solution, wherein a common control unit calculates the timing of output voltage state change instances and sends this timing signal to the control units of each parallel connected inverters. The inverter-specific control units then form the control signals of power stage IGBT's essentially simultaneously. Simultaneous switching moments boost equal load share, but in this method and arrangement the signal delay tolerances of the components in the signal path may cause deviation from the ideal situation and demand to use a derating factor and great output impedances. In the patent publication U.S. Pat. No. 8,432,714 a solution is presented, wherein the load balancing between the parallel connected inverter modules is based on adjusting the switching instructions of switch components according to the measured output currents and measured inverter module temperatures.